The present invention relates generally to a high-frequency coil system for a NMR (nuclear magnetic resonance) imaging apparatus, and more particularly to a NMR high-frequency coil system implemented in a divisible structure for increasing a filling factor and enhancing manipulatability of the coil system.
The imaging apparatus realized by utilizing the nuclear magnetic resonance or NMR phenomenon (hereinafter referred to as MRI apparatus in abbreviation) is used for obtaining diagnosis information of morbid growths such as cancer or the like of the human body.
In the MRI apparatus, a permanent magnet or a coil-type magnet is employed. In view of heavier weight of the parmanent magnet system, the coil-type magnet system is widely used.
Operation of placing the body to be diagnosed in the static magnetic field generated by the magnet and taking in and out the body therefrom is commonly carried out in a horizontal plane in consideration of facilitation in moving the body lying on a movable table.
In the case of a vertical-field type MRI apparatus in which the static magnetic field is generated in the vertical direction by employing the coil-type magnet, difficulty is encountered more or less in moving the body to be diagnosed in and out of the static magnetic field. For this reason, a horizontal-field type MRI apparatus in which the static magnetic field is generated in the horizontal direction is generally adopted.
In the MRI apparatuses mentioned above, a high-frequency coil is indispensably required for applying a high-frequency magnetic field to the body under inspection for exciting the nuclear magnetic resonance and detecting a high-frequency magnetic field representative of the nuclear magnetic resonance signal resulting from the nuclear magnetic resonance. For such high-frequency coil, there are known two types, i.e. a solenoid-type coil and a saddle-type coil. In view of the intrinsic nature of the nuclear magnetic resonance phenomenon, it is prerequisite that the direction in which the high-frequency magnetic field is applied must be orthogonal not only to the direction of the high-frequency magnetic field generated by the nuclear magnetic resonance but also to the direction of the static magnetic field. In order to meet this condition, the solenoid-type coil, if to be employed, would have to be disposed orthogonally to the static magnetic field. In that case, however, it becomes practically impossible to move the body or object to be inspected in and out of the static magnetic field. In contrast, the saddle-type coil is evaded from this sort of problem and is thus employed commonly.
At this juncture, it is believed that if the high-frequency coil could be implemented in such a structure that it is divisible or separable, a feeling of uneasiness or a sense of oppression of a patient could be mitigated when he or she is moved into and out of the apparatus. More specifically, it is desirable that the coil is divided or separated before the patient is placed in the apparatus for facilitating the positioning of a part to be diagnosed of the body and again unified after the positioning, because then the patient could feel free from the uneasiness and the oppression. On the other hand, in order to enhance the sensitivity, it is desirable to increase the filling factor which is given by the ratio of the volume of the body under inspection to that of the coil system. As an attempt for satisfying the conditions mentioned above, it is known in conjunction with the saddle-type coil to realize it divisible in the vertical direction. Reference may be made to, for example, Japanese Patent Application Laid-Open No. 150738/1985 (JP-A-60-150738). With this known structure of the saddle-type coil, positioning of the body for the diagnosis can certainly be facilitated.
It is further noted that the permanent magnet is more advantageous than the coil-type magnet in view of the low running cost and low leakage magnetic field. Besides, at the present state of the art, the permanent magnet can be improved considerably in the weight as well and thus attracts attention as the magnet for generating a magnetic field of relatively low intensity.
When the permanent magnet is employed, the vertical field type coil system in which the static magnetic field is generated is preferred from the standpoint of positioning of the body to be diagnosed. In the vertical field type system, the use of the solenoid-type coil is desirable which is more sensitive when compared with the sensitivity of the saddle-type coil, because than the high-frequency magnetic field generated by the solenoid-type coil or the high-frequency magnetic field detected by it can be orientated orthogonally to the static magnetic field.
However, in view of the fact that the solenoid-type coil is realized by winding it about the horizontal axis, it will be difficult to divide or separate the coil into two halves in the vertical direction. This is the reason why the vertically divisible or separable solenoid-type coil could not be realized heretofore notwithstanding a high demand existing therefor. In other words, the problem lies in that although the solenoid-type high-frequency coil can be so implemented that the axis thereof extends horizontally when the solenoid-type coil is employed in combination with the parmanent magnet in the vertical-field type MRI system, difficulty is encountered in realizing the solenoid-type high-frequency coil to be divisible or separable.